Field of the Invention
The invention relates to a method for laser welding of one or more workpieces made from press hardenable steel, particularly manganese-boron steel, in a butt joint, in which the workpiece or the workpieces have a thickness of at least 1.8 mm and/or a jump in thickness of at least 0.4 mm arises at the butt joint, and in which the laser welding takes place with the supply of filler wire into the molten bath generated with a laser beam.
Description of Related Art
Tailored blanks made from sheet steel are used in the automotive industry to fulfil high demands on the crash safety with the smallest possible bodywork weight. For this purpose, individual blanks or strips of different material quality and/or sheet thickness are joined together by laser welding in a butt joint. In this manner, various points of the finished bodywork component can be adapted to different loads. Thus, at locations with high loading, thicker or higher strength sheet steel can be used and thinner steel sheets or else sheets made from relatively weak deep-drawing grades can be used in the remaining locations. Additional reinforcing parts on the bodywork become superfluous due to such tailored sheet blanks. This saves material and enables the reduction of the total weight of the bodywork.
In recent years, boron-alloyed steels, particularly manganese-boron steels have been developed, which achieve high strengths, for example tensile strengths in the range of 1500 to 2000 MPa when hot formed with rapid cooling. In the initial state, manganese-boron steels typically have a ferritic/pearlitic structure and have strengths of approx. 600 MPa. By press hardening, i.e. by heating to austenising temperature and subsequent rapid cooling in the compression mould, a martensitic structure can be set however, so that the thus-treated steels can achieve tensile strengths in the range from 1500 to 2000 MPa.
The bodywork components, for example B pillars, produced from such tailored steel blanks have a flawless hardness profile up to a certain sheet thickness or a certain thickness jump. However, it was determined that at a sheet thickness larger than or equal to approx. 1.8 mm, particularly larger than or equal to approx. 2.0 mm, or a thickness jump greater than or equal to approx. 0.4 mm, the problem occurs that the laser weld seam does not harden sufficiently during hot forming (press hardening). Then, a martensitic structure only results to a certain extent in the weld seam, so that during the loading of the finished component, a failure may occur in the weld seam. This problem is presumably associated with the fact that, particularly in the case of a thickness jump, sufficient contact to the cooled forming tool or cooling tool cannot generally be ensured and as a result, the weld seam cannot be completely converted into martensite.
A laser-arc hybrid welding method is described in US 2008/0011720 A1, in which method blanks made from manganese-boron steel, which have an aluminium-containing surface layer, are connected to one another in a butt joint, the laser beam being combined with at least one electric arc, in order to melt the metal at the butt joint and to weld the blanks to one another. The electric arc is in this case output by means of a tungsten welding electrode or is formed at the tip of a filler wire if a MIG welding torch is used. The filler wire can contain elements (e.g. Mn, Ni and Cu), which induce the transformation of the steel into an austenitic structure and benefit maintenance of the austenitic transformation in the molten bath.
Using this known laser arc hybrid welding method, it should be achieved that hot formable blanks made from manganese boron steel, which are provided with a coating with an aluminium/silicon basis, can be welded without preceding removal of the coating material in the region of the weld seam to be produced, wherein it should nonetheless be ensured however that aluminium located at the abutting edges of the blanks does not lead to a lowering of the tensile strength of the component in the weld seam. By providing an electric arc behind the laser beam, the molten bath should be homogenised and as a result, local aluminium concentrations larger than 1.2% by weight, which create a ferritic structure, are eliminated.
This known hybrid welding method is relatively expensive with regards to the energy consumption, owing to the generation of the electric arc.